1. Field of the Invention
This invention relates to the technique of suppressing the influence of the contamination of exposure atmosphere, which is suitable for use in the manufacture of a device having a fine pattern such as semiconductor devices.
This invention is particularly suitable for an exposing apparatus for effecting exposure by the use of light of a short wavelength (0.5 to 50 nm) like EUV light, or an exposing apparatus for effecting exposure under a high vacuum atmosphere by the use of an optical element such as a mirror or a lens.
2. Description of Related Art
In recent years, in the optical lithography technique for manufacturing semiconductors, the shortening of the wavelength of exposure light has been advanced and progress has been made from i-line and g-line to KrF excimer laser and ArF excimer laser. If the shortening of the wavelength of exposure light progresses, it will become possible to transfer a minuter mask pattern to a wafer. However, to exposure a pattern of a thin line width, lithography using ultraviolet light is limited in principle. So, EUV lithography using extreme ultraviolet light (EUV light, 13 to 20 nm) of a shorter wavelength than ultraviolet light has been drawing attention.
A typical wavelength used in the EUV light is 13.5 nm and therefore, it is possible to realize resolution for exceeding that of the optical lithography hitherto, on the other hand, the EUV light has the nature that it is readily absorbed by substance. Therefore, if reduction exposure using a refractive optical system is effected as in the conventional lithography using ultraviolet light as a light source, the EUV light will be absorbed by a glass material and the quantity of light reaching a member to be exposed such as a wafer will become extremely small. Therefore, when exposure is to be effected by the use of the EUV light, it is necessary to configure reduction exposure using a reflecting optical system.
FIG. 4 is a schematic view of a conventional reduction projection exposing apparatus using the EUV light (see Japanese Patent Application Laid-Open No. 2003-45782). An EUV exposing apparatus 200 has an EUV light source 210, an illuminating optical system 220, a reticle (mask) 230, an alignment optical system 240, a reticle stage 250, a wafer stage 260, a wafer 270, a vacuum container 280, a reflection type reduction projection optical system 100, a first mirror 110, a second mirror 120, a third mirror 130, a fourth mirror 140, a fifth mirror 150 and a sixth mirror 160, and is also provided with an exhausting system, not shown, for exhausting gas in the vacuum chamber 280, etc.
There are several kinds of EUV light source, and a laser producing plasma light source which is one of them can emit light of only a substantially necessary wavelength band by the selection of a target material. When for example, Xe is ejected as a target material from a pulse nozzle, and a pulse laser is applied thereto to thereby generate plasma, EUV light of a wavelength 13-14 nm is emitted.
The illuminating optical system is comprised of a plurality of multi-layer film mirrors, an optical integrator, etc. As the roles of the illuminating optical system, mention may be made of efficiently condensing light emitted from the light source, and uniformizing the illuminance in an exposure area. Also, the optical integrator has the role of uniformly illuminating the mask at a predetermined numerical aperture.
The projection optical system is comprised of a reflecting optical system using a multi-layer film mirror alternately coated with Mo and Si. This Mo/Si multi-layer film can obtain normal-incidence reflectance of the order of 67% in the vicinity of a wavelength 13 nm. It is difficult in principle to provide reflectance of 100% and most of absorbed energy changes into heat. Therefore, low thermal expansion glass or the like is used for the base material of the mirror. In the reflecting optical system, a plurality of such Mo/Si multi-layer film mirrors are used for aberration correction, however, to keep the transmittance of the EUV light, it is necessary to minimize the number of the multi-layer film mirrors.
The reticle stage and wafer stage of the EUV exposing apparatus have mechanisms for driving under a vacuum environment, and the reticle stage and the wafer stage scan in synchronization with each other at a speed ratio proportional to a reduction magnification. Also, the positions and postures of the reticle stage and the waver stage are observed and controlled by a laser interferometer, not shown.
A reticle held by a reticle chuck and a wafer held by a wafer chuck are highly accurately positioned by fine motion mechanisms carried on the reticle stage and the wafer stage.
The alignment optical system is an apparatus for detecting the positional relation between the position of the reticle and the optical axis of the projection optical system, and the positional relation between the wafer and the optical axis of the projection optical system. Thereby, the positions and angles of the reticle stage and the wafer stage are set so that a projected image may be applied to a predetermined position on the wafer. Also, a focus position in a direction perpendicular to the surface of the wafer is detected by a focus detecting mechanism, and the position and angle of the wafer stage are controlled, whereby the imaging position on the surface of the wafer is always kept.
In order to avoid the absorption of the EUV light by substance, it is necessary that the space, to which the EUV light of the EUV exposing apparatus, is to be kept in vacuum. Therefore, a plurality of exhaust systems such as vacuum pumps are mounted on the exposing apparatus.
The EUV light used in the EUV exposing apparatus is absorbed by the atmosphere in the apparatus. Particularly, oxygen and moisture strongly absorb the EUV light. Therefore, to keep the transmittance of the EUV light high, it is necessary to render the interior of a chamber into a vacuum state by the utilization of a vacuum pump or the like. It is desirable that the pressure in the chamber through which the EUV light passes be 10−3 Pa or less and the partial pressure of oxygen and moisture be infinitely low. However, some of moisture adhered to the wafer during the conveyance of the wafer is diffused in the chamber. Further, moisture is liable to adhere to the inner wall of the chamber and is difficult to be exhausted. The adherence of moisture to the optical element becomes a cause of the oxidization of the optical element and the reduction of the reflectance of the optical element.
Also, when the interior of the chamber becomes a vacuum state, hydrocarbon is produced from the mechanism portions of the stages or the like. Further, reaction of resist by the exposure light during exposure also produces hydrocarbon, and when such hydrocarbon is applied to the exposure light on the surface of the optical element, it adheres as carbon to the surface of the optical element. The carbon adhered to the optical element absorbs the EUV light and reduces the reflectance of the optical element. The reduction of the reflectance of the optical element leads to a reduction in throughput.
Thus, particularly in the space where the optical element in the EUV exposing apparatus is installed, the partial pressure of moisture and hydrocarbon need be kept low.
To lower the partial pressure of moisture and hydrocarbon or the like in the exposing apparatus, the step of enhancing the capability of the exhaust system such as a vacuum pump is also effective. However, it is inevitable that the moisture adhering to the conveyed wafer and the hydrocarbon produced from the resist and the mechanism portions of the stages drift in the space, where the optical element is installed, due to diffusion and therefore, it is difficult to improve the throughput.
Also, at present, regarding the manufacture of semiconductor devices such as DRAM and MPU, research and development are being energetically made to realize a device having a line width of 0.1 μm or less in terms of the design rule. As an exposing apparatus for use in this generation, an exposing apparatus using the extreme ultraviolet light (EUV) is regarded as being potent. Since in the EUV exposing apparatus, exposure in the atmosphere is impossible, exposure cannot help being effected in vacuum.
In the EUV exposing apparatus, particles are produced by sliding movement and friction such as the operations of a robot hand and a gate value until a reticle and a wafer are conveyed to an apparatus chamber, and there has been the possibility of these particles adhering to the reticle and the wafer. Also, there has been the possibility that particles produced from movable portions such as stages and the like may adhere to the surfaces of the reticle and the wafer during exposure.
Such adherence of the particles to the surfaces of the reticle and the wafer has caused the problem the yield of the manufacture of the device and the reliability of the device are lowered. Particularly, if the particles adhere to the circuit pattern surface of the reticle, the particles will be transferred to the entirely same position at each shot in actual exposure. This has led to the problem that the yield of the manufacture of the device and the reliability of the device are greatly lowered.